Variable-ratio hydraulic steering system

ABSTRACT

The invention changes over to another steering ratio automatically and very safely on the transition from servo to emergency operation. In the steering system, only a few of the lines (13) link the metering pump (10) to one of the cylinder connections (7, 8) and in each of the other lines (13&#39;) there is a changeover unit (16) which also links the lines (13) to the cylinder connections (7, 8) or the delivery connection (6) depending on the method of operation. These steering systems are used in mobile systems, especially slow-moving vehicles.

BACKGROUND OF THE INVENTION

The invention relates to a hydraulic steering system.

Such steering systems are employed in particular in slow-moving vehicleswith high axle loads.

Steering systems of this kind have been known for a long time and areemployed in a multitude of embodiments.

The steering systems comprise mainly a rotary slide control valve and anorbital displacement system. The rotary slide control valve is formed byan outer control sleeve, fitted into the casing, and an innerconcentrically disposed control piston. The control piston is connected,on the one hand, by a gear hub to the steering wheel and, on the otherhand, relatively movable by a pin connection to the control sleeve. Thecontrol sleeve is rigidly connected to the rotor gear wheel of theorbital displacement system with a pin connection and a drive shaft. Thecontrol piston is centered in its position by a way of a spring elementrelative to the control sleeve. The control piston and the controlsleeve are rotatable relative to each other by a limited amount againstthe force of this spring element. A hydraulic connection exists betweenthe rotary slide control valve and the orbital displacement systemthrough commutator boreholes.

The hydraulic steering system is furnished with connections for a feed,for a return, and for two cylinder lines for the control cylinder.

These steering devices are characterized by a high volume ofdisplacement without translation ratio between servo operation andemergency operation.

This means, however, that in case of a possible failure of the supplypump the steering system switches instantaneously to emergencyoperation, and that therefore all of the required steering power has tobe supplied manually at the steering wheel. This is a safety risk inparticular in road traffic.

Therefore, it has been a long-time endeavor to provide steering systems,which switch automatically to another translation ratio in the concretesituation of the change from servo operation to emergency operation.

Thus, the German printed patent document DE 22 28 531 C2 is known, whichshows the possibility for changing the displacement volume for eachrotation of the steering wheel.

This steering system is furnished with an orbital displacement systemwith two parallel-connected displacement units, wherein the movable gearwheels of the displacement unit are mechanically connected to each otherby a gear shaft. Switching valves are disposed in the hydraulicconnection lines between the two displacement units, wherein theswitching valves connect the two displacement units in the presence ofcompression oil, i.e. in the servo steering operation, and render adisplacement unit ineffective in emergency steering operation, i.e. incase of failure of the supply pump.

The number of the necessary rotations of the steering wheel for reachinga certain locking angle or steering lock of the steered wheels becomesnecessarily higher based on the decreased displacement volume. However,the required steering power remains substantially constant in case of acorresponding construction of the steering and thus manageable by theoperator of the vehicle.

However, the constructive expenditure and thus the cost expenditure isrelatively large. Based on the complicated structure, the steeringsystem is very much subject to interferences. Thus, the steering systemdoes not eliminate the safety risk in road traffic.

A rotary piston machine has become known from the German printed patentdocument DE 21 40 569, which rotary piston machine operates inparticular as an engine. The rotary slide control valve of the rotarypiston machine is furnished with a switching device, wherein theswitching device connects selectively each fourth of twelve valveopenings of the outer control sleeve to the feed or to the discharge.

Thus, the rotary piston machine operates in one of the two operationalstages with a decreased chamber volume on the pressure side and thuswith a decreased torque and with an increased rotational speed. Theswitching device is operated manually.

This solution is unsuitable for steering devices because a manualoperation of the switching device is completely unsuitable.

In addition, there is another safety risk, since the safety of theswitching from servo operation to emergency operation depends on thecontinuous functional safety and readiness for operation of theswitching device.

OBJECTS OF THE INVENTION

Therefore, there exists the object to provide a steering device of therecited kind which, in case of an emergency operation, switchesautomatically and with a high level of certainty to another translationratio.

The invention eliminates the recited disadvantages of the state of theart. A substantial advantage is the high safety and certainty of theswitching to a different translation ratio. Based on the presence ofseveral switching units, it is only of little importance, if possiblyone of the switching units fails, since in this case a change intranslation ratio is also assured based on the remaining switchingunits, even if it is not the optimum translation ratio.

BRIEF DESCRIPTION OF THE INVENTION

The invention is further described in the following by way of twoembodiments.

There is shown for this purpose:

FIG. 1: A first embodiment with a steering system in open-centerconstruction

FIG. 2: A second embodiment with a steering system in open-centerconstruction

FIG. 3: A third embodiment with a steering system in open-centerconstruction.

DETAILED DESCRIPTION

The hydraulic circuit is illustrated in simplified version for reasonsof viewability and comprises substantially a tank 1, a supply pump 2, asteering device 3, and a control cylinder 4 acting on the wheels of thevehicle.

The steering device 3 is furnished with a feed connector 5, a dischargeconnector 6, and two cylinder connections 7 and 8. Main elements of thesteering device 3 are a control valve 9 and a metering pump 10,operating according to the orbital principle, with an outer ring of, forexample, seven teeth and with a runner gear wheel with then six teeth.The control valve 9 is fitted into a casing and is conventionallyfurnished with an inner control piston, controllable by a steering wheel11, and an outer control sleeve, mechanically connected to the rotor ofthe metering pump 10. The control piston and the control sleeve aredisposed concentrically to each other and are rotatable to a limiteddegree and against the force of a spring. The control piston isfurnished with six parallel-acting lines 12, distributed uniformly atthe circumference of the control piston. The lines 12 have a connection,on the one hand, to the feed connector 5 and, on the other hand, to theinput side of the metering pump 10. The control piston is furnished witha total of six lines 13 and 13', wherein the lines 13 and 13', on theone hand, are connected with the output side of the metering pump and,on the other hand, to a cylinder connection 7 or 8. The six lines 12form six adjustable input throttles 14 in cooperation with the channelsof the control sleeve. A short-circuit path branches from the feedconnector 5, disposed in front of the input throttles 14 of the controlvalve 9, to the discharge connector 6, in which a short-circuit throttle15 is disposed, and which behaves inversely proportional to the inputthrottles 14.

According to the first embodiment according to FIG. 1, three lines 13lead directly to the respective cylinder connection 7 or 8. In eachcase, an externally controlled switching unit 16 is disposed in thethree remaining lines 13'. The switching unit 16 is preferablyconstructed as a 3 by 2-way valve with a second output-side connectionto the discharge connector 6 or to the feed connector 5. The switchingunit 16 is furnished with a hydraulic adjustment unit which, on the onehand, is connected to the feed line, disposed upstream relative to themetering pump 10 and, on the other hand, is connected to one of thelines 13 or 13', disposed downstream relative to the metering pump 10.Because of the higher pressure differential, the adjustment unit of theswitching unit 16 is advantageously connected to one of the lines 13.

The switching units can also cooperate in a conventional way withdifferent kinds of adjustment units, for example, with in each case anelectrical adjustment unit.

It is further possible to dispose in each case an externally-controlledshut-off valve in the control line, connected to the line 13, of one orseveral switching units 16. The vehicle operator can preprogram with theexternally-controlled shut-off valve the translation ratio during theswitching from servo operation to emergency operation as required basedon the blocking of the individual switching units 16.

According to the second embodiment according to FIG. 2, a switching unit16 is disposed outside of the lines 13', capable of being switched off,in such a way that a collector line 17 leads to one or several lines 13'on the input side, and that, as desired, in each case a connection ispresent to one of the cylinder connections 7 or 8 or to the dischargeconnector 6 on the output side.

The output side can also be connected to the feed connector 5 instead ofbeing connected to the discharge connector 6. The adjustment unit of theswitching unit 16 is again connected, on the one hand, to the feed linedisposed in front of the metering pump 10 and, on the other hand, withone of the lines 13 or 13' disposed after the metering pump 10. Otherkinds of adjustment units can be employed also in this case. A checkvalve 18, opening in the direction to the respective cylinder connection7 or 8, and disposed downstream after a branch leading to the switchingunit 16, is disposed in each line 13'.

As shown in FIG. 3, it is also advantageous to employ a check valve 19,responsible for all lines 13', instead of the individual check valves18. In this case, a 2 by 2-way valve is employed as a switching unit16', wherein the 2 by 2-way valve can again be controlled in differentways.

In case of a non-actuated steering device 3, the control sleeve and thecontrol piston of the control valve 9 are disposed in a neutral positionsuch that all six input throttles 14 are closed and that theshort-circuit throttle 15 is open. The oil, delivered by the supply pump2, flows back unused through the short-circuit throttle 15 to the tank1.

Upon actuation of the steering wheel 11, the control piston is deflectedrelative to the control sleeve and thus the input throttles 14 areopened. The oil passes into the metering pump 10, is metered in themetering pump 10, and is transported in a metered amount into the lines13 and 13' leading to the control cylinder 4. The oil passes immediatelyto one of the cylinder connections 7 or 8 through the three lines 13 andthus to the control cylinder 4. In the other three lines 13', the oilpasses in each case through the opened switching units 16, 16'.

The switching units 16, 16' connect the lines 13' to one of the cylinderconnections 7 or 8 in servo operation, because the control pressure infront of the metering pump 10 surpasses the control pressure after themetering pump 10 by the amount of the flow pressure losses occurringbetween the two pressure monitoring positions.

The line at the feed connector 5 is without pressure in an emergencyoperation, that is when the supply pump 2 is switched off. The oilnecessary for the steering device is in this case supplied solelythrough the manual activation of the steering wheel 11 and thus by themetering pump 10 and is fed to the control cylinder 4.

The oil passes in this case again through the three lines 13 directly toone of the cylinder connections 7 and 8 and thus to the control cylinder4. The switching units 16, 16'are switched and connect the respectivelines 13' to the discharge connector 6 or, alternatively, to the feedconnector 5 because at this point in time the control pressure ratio ofthe pressure in front of the metering pump 10 and of the pressure afterthe metering pump 10 is inverse relative to the servo operation. In thiscase, only three of the six lines branching from the metering pump 10are participating in the oil transfer, whereby the steering power is cutin half and the number of the required rotations at the steering wheel11 for achieving the same deflection of the steered wheels is doubledrelative to the servo operation.

We claim:
 1. Hydraulic steering device with translation ratio, whichcomprises cylinder connections, a control valve, and a metering pump,wherein the control valve includes a control piston, controllable by asteering wheel, and a control sleeve, mechanically connected to a rotorof the metering pump, and wherein the control piston and the controlsleeve are furnished with channels cooperating and communicating witheach other and with the metering pump, and wherein the metering pump hasseveral displacement chambers, of which displacement chambers severalare continuously connected to one of the cylinder connections, and theremaining displacement chambers are connected by a switching unit toeither one of the cylinder connections or to a dischargeconnector,characterized in that a metering pump (10) is employed whichhas exclusively parallel-disposed displacement chambers with first andsecond lines (13, 13') of which first lines (13). one or several of thefirst lines (13), however at the most one less than the number of linespresent, connect the metering pump (10) directly to one of the cylinderconnections (7, 8), wherein the switching unit (16) is coordinated tothe remaining second lines (13'), and wherein an adjustment unit of theswitching unit (16), on the one hand, is subjected to the pressurepresent upstream of the metering pump (10), in the position connectingthe metering pump (10) to one of the cylinder connections (7, 8) and, onthe other hand, to the pressure present downstream of the metering pump(10), in the position connecting the metering pump (10) to the dischargeconnector (6) or to a feed connector (5).
 2. Hydraulic steering deviceaccording to claim 1, characterized in that the adjustment unit of theswitching unit 16 is subjected to the pressure prevailing in the firstlines (13).
 3. Hydraulic steering device according to claim 2,characterized in that a switching unit (16) is coordinated to eachsecond line (13').
 4. Hydraulic steering unit according to claim 3,characterized in that three second lines (13') are provided. 5.Hydraulic steering unit according to claim 4, characterized in that ineach case an externally-controlled shut-off valve is disposed in thecontrol line of one or several switching units (16), wherein the controlline is subjected to the pressure present downstream of the meteringpump (10).
 6. Hydraulic steering device according to claim 2,characterized in that the switching unit (16, 16') is disposed outsideof the second lines (13'), which are disconnectable, and is connected onthe input side with a collector line (17) to the other second lines(13'), and wherein a first check valve (18), opening in direction to oneof the cylinder connections (7, 8) and disposed downstream after thebranch leading to the switching unit (16, 16'), is disposed in each ofthe second lines (13').
 7. Hydraulic steering unit according to claim 2,characterized in that the switching unit (16') is disposed outside ofthe second lines (13'), which are disconnectable wherein the switchingunit (16') is connected on the input side with a collector line (17) tothe other second lines (13'), and wherein a second check valve (19) iscoordinated to all first lines (13) in place of a first check valve(18).
 8. Hydraulic steering device according to claim 6, characterizedin that the switching unit (16, 16') is furnished with a pneumaticadjustment unit.
 9. Hydraulic steering device according to claim 6,characterized in that the switching unit (16, 16') is furnished with anelectrical adjustment unit.
 10. Hydraulic steering device according toclaim 6, characterized in that the switching unit (16, 16') is furnishedwith an electromagnetic adjustment unit.
 11. Hydraulic steering deviceaccording to claim 6, characterized in that the switching unit (16, 16')is furnished with a mechanical adjustment unit.
 12. Hydraulic steeringdevice according to claim 11, characterized in that the mechanicaladjustment unit is furnished externally controlled.
 13. Hydraulicsteering device according to claim 5, characterized in that theswitching units (16) are furnished with pneumatic adjustment units. 14.Hydraulic steering device according to claim 5, characterized in thatthe switching units (16) are furnished with electrical adjustment units.15. Hydraulic steering device according to claim 5, characterized inthat the switching units (16) are furnished with electro-magneticadjustment units.
 16. Hydraulic steering device according to claim 5,characterized in that the switching units (16) are furnished withmechanical adjustment units.
 17. Hydraulic steering device according toclaim 16, characterized in that the mechanical adjustment units arefurnished externally controlled.